Gertz’s reports suggest that China would be testing an ASAT weapon against navigation and reconnaissance satellites at high altitudes, but doesn’t give specifics about which satellites would be the targets nor what kind of ASAT about it would be.

But let’s take a closer look at that statement about the 24 ASAT weapons, and what strategic use a higher altitude weapon really might provide. Currently, the planned minimum constellation size of the GPS system is 24 well-placed satellites in semi-synchronous orbits. The current GPS constellation has 30 active satellites. While accuracy improves as the number of visible satellites increases, for basic service a user needs a minimum of four satellites in view if s/he has no other source of information on position or timing. (For a typical user, several more satellites will be in view most of the time.)

GPS services degrade gradually

GPS is a very robust system, however, and service degrades very gradually as it loses satellites from the constellation. An adversary would need to compromise 6 satellites before the constellation shrunk to the design minimum of 24. Yet more satellites would need to be turned off before the attack had any immediate effect on military operations. Looking at scenarios for ASAT attack on GPS satellites, Geoff Forden estimates that even with just 12 satellites left in the constellation, an adversary can only hamper the U.S. ability to use precision-guided munitions for a few hours per day. Moreover, those aren’t just any satellites—in Forden’s model, the adversary chose well and selected the 12 remaining satellites to give the worst GPS performance over Beijing. This degradation in performance would require the U.S. to lose 18 GPS satellites.

First, it is very challenging to target more than one GPS satellite at a time. The 24 GPS satellites are arranged in six widely separated orbital planes with four satellites in each.(plus the spares). The planes are separated by 55 degrees and so each plane would require a separate ASAT weapon. The four satellites in each plane have slight phasing irregularities that make it difficult for a “looping” ASAT with perigee near the earth and apogee at the constellation’s altitude, to pick off the satellites in each plane one by one.

GPS satellites are hardened against attack by lasers. The satellites are spread widely enough in space that even a nuclear weapon ASAT could only be expected to take out one satellite at a time, and the satellites are designed to operate in the high-radiation environment of the Van Allen belts’ accumulated radiation.

High altitude of GPS satellites puts them out of reach of many missiles

It would take a serious and time-consuming effort to destroy enough satellites to interfere with GPS. The 20,000 km altitude of semi-synchronous orbit puts it out of reach of all but the most advanced ICBMs. Recall the “one-half rule”: a missile with range R with a given payload mass can lift that mass to approximately altitude R/2 (see Section 8, Appendix B of The Physics of Space Security). While these missiles would not need to be as powerful as space launch vehicles that put satellites in MEO orbits, they would be large and relatively expensive.

Putting an ASAT into a looping orbit (see above) or the same orbit as a set of GPS satellites seems like a remote possibility. Using these large launch vehicles to get at any kind of useful number of satellites would stress any country’s launch capabilities, as the number of launch pads capable of space launch is limited. In 2011, China’s banner year for space, it launched a total of 19 times. Should an attack on GPS begin, the U.S. would be on notice and could have GPS satellites start making small maneuvers that would be enough to evade attack.

Most countries that can reach GPS orbits use GPS orbits

The United States has 30 GPS satellites in semi-synchronous orbits and Russia has 32 Glonass navigation satellites there. In April and September of 2012, China launched four satellites to this orbit, part of its own planned satellite navigation system, Beidou. While MEO is not as densely populated as low earth orbits, these satellites represent significant financial and capability investments. Destructive, debris-producing tests in these orbits would come at a high cost even to those who did them.

The stories about a test are speculative, and it is unclear what is in the works, if anything. High-altitude could also mean geosynchronous orbits, at much higher 36,000 km altitudes. In a future post, I will take a look at that.

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